In aerospace and other industries it may be occasionally necessary to inspect structures for the presence of irregularities in the structures. Ultrasonic inspection is a technique which is extensively used to inspect composite and other structures for irregularities, since the irregularities tend to be laminar in nature and located parallel to the overlying sound-entry interface (an optimum orientation for ultrasonic detection). However, ultrasonic inspection of irregularities beneath angled interfaces such as chamfers and countersunk surfaces may not be accurate since those types of surfaces tend to refract and scatter the ultrasonic beams which are emitted through the interfaces. Flush fastener holes, existing in the thousands on a typical aircraft, represent a common angled interface, one that is subject to inspection to identify damage caused by drilling or fastener removal.
A variety of methods have been devised to minimize the undesirable effects of refraction and scattering which are caused by angled interfaces in a structure subjected to ultrasonic inspection. One method includes positioning of ultrasonic transducers parallel to the surface of a composite structure and at an angle to the plies of the structure. The ultrasonic beam enters the structure and is channeled by the fibers in the plies. Laminar irregularities in the plies do not return an ultrasonic echo, thus failing to reveal the presence of the irregularities.
Another method involves positioning of ultrasonic transducers parallel to the plies in the structure on a plastic wedge. Due to the velocity difference between the plastic wedge and the carbon fiber material, the ultrasonic beam refracts at an angle to the plies. Laminar irregularities in the plies do not return an ultrasonic echo, thus failing to reveal the presence of the irregularities.
Some solutions have entailed shaped ultrasonic arrays to sweep sound beams through the material. However, these techniques may still produce marginal echo returns from delaminations in the plane of the plies. In other cases, ultrasonic inspection can be performed from a parallel back surface toward the angled surface, but in many such cases, access to the back side of the structure is not possible.
Over the years, efforts have been made without success to identify a material that matches the acoustic characteristics of carbon fiber laminate. Due to the ultrasonic velocity and acoustic impedance mismatch between those materials and carbon fiber laminate, those materials will cause refraction and interface energy loss when used as a geometry compensator. With some materials, a suitably-chosen angled transducer interface can result in a perpendicular beam in the laminate, however the acoustic impedance losses due to dissimilar materials at the interface remain. It has been found that a CFRP (Carbon Fiber Reinforcement Polymer) wedge-shaped or plug-shaped transducer attachment essentially restores the structure being inspected to a plate-like configuration, thereby greatly simplifying ultrasonic inspection. The bond line interface between the wedge or plug attachment and the structure has a negligible effect on the ultrasonic beam since there is no refraction or acoustic impedance loss.
Therefore, geometry compensating transducer attachments which eliminate or reduce refraction and scattering in ultrasonic inspection of structures having chamfers or countersunk surfaces are needed.